The invention relates to a method and apparatus for machining by electrical discharges an electrode workpiece by way of an electrode tool, wherein consecutive voltage pulses are applied between the electrodes. More particularly, the invention relates to an EDM method and apparatus permitting to develop a variable magnitude which is a characteristic of the machining performance of efficiency factor or index, and to automatically vary step by step one of the machining conditions or parameters such as to operate at the maximum value of the machining performance index.
The cut-off time interval between consecutive voltage pulses, in an EDM apparatus, is an important machining parameter which is known to affect the machining efficiency and which, when properly utilized, permits to obtain maximum machining efficiency. The performance factor or index or, in other words, the machining efficiency, is capable of quantitative representation as a function of any one of a plurality of machining conditions or parameters such as, for example the machining parameter corresponding to the total duration of all the effectively machining electrical discharges within a predetermined time frame. When the length of the cut-off time interval between two consecutive voltage pulses is decreased, a corresponding decrease of the number of effective electrical discharges is observed due to a downgrading of the quality of the electrical discharges, and consequently of the machining efficiency. When, on the contrary, the duration of the cut-off time interval is increased, a corresponding decrease of the number of effective electrical discharges during the same time frame is observed, with the result that the machining efficiency tends to also decrease. Between these extremes in cut-off time interval duration, there exists a zone of operation in which the machining efficiency, or the over-all performance index of the EDM apparatus, reaches a maximum value.
Through the utilization of appropriate logic circuitry, it is possible to provide the machine operator with a visual display of a magnitude representative of the machining efficiency, or to utilize an electrical magnitude representative of the machining efficiency for automatically controlling and modifying the machining parameters such as to optimize the machining conditions as, for example, disclosed in U.S. Pat. No. 4,090,961, assigned to the same assignee as the present application. The speed at which the apparatus corrects itself, so as to be self-adaptative, can yet be improved as provided by the present invention, by automatically varying step by step the duration of the cut-off time interval between consecutive pulses, and by multiplying the difference or increment between consecutive steps by the slope of the representative curve of the machining efficiency in function of the duration or length of the cut-off time interval.
A good approximation of the value of the slope results from effecting the quotient of the change in machining efficiency by the corresponding step increment of the variation in duration of the cut-off time interval that originally caused the resulting change in machining efficiency. In this manner, the increment steps by which the duration of consecutive cut-off time intervals is varied are small when operating proximate the top of the machining efficiency representative curve, and the step increments are wider when operating at a portion of the representative curve distant from its top. However, it has been observed that if there occurs an accidental incursion into a critical zone corresponding to very short cut-off time intervals that cause the representative curve to have a positive slope, an increase of the duration or length of the cut-off time interval does not permit to achieve immediately a substantial improvement of the machining efficiency. In other words, it has been observed that excessive degradation of the machining conditions gives rise to a downgrading phenomenon resembling hysterisis which impedes rapid re-establishment of favorable machining conditions.
The process of the present invention aims at avoiding the recurrent frequent appearance of such a hysterisis phenomenon by providing a determination of the sign of the quotient of the variation of the magnitude representing the machining efficiency by a corresponding change in the machining parameter causing the change in efficiency, and by providing the step increments by which the machining parameter is varied or changed a first value when the resulting quotient is positive and at least one second value, greater than the first, when the resulting quotient is negative.